The reality of software products is that they are in nearly in all cases developed/maintained over time, though--and whenever that's the case, the black box metaphor fails. It's an idealization that only works for single moments of time, and yet software development typically extends through the entire period during which a product has users.

> I read OPs "good code" to mean "highly aesthetic code" (well laid out, good abstractions, good comments, etc. etc.)

The above is also why these properties you've mentioned shouldn't be considered aesthetic only: the software's likelihood of having tractable bugs, manageable performance concerns, or to adapt quickly to the demands of its users and the changing ecosystem it's embedded in are all affected by matters of abstraction selection, code organization, and documentation.

> I wouldn't care so long as I wasn't expected to maintain it.

But, if you’re the one putting out that software, of course you will have to maintain it! When your users come back with a bug or a “this flow is too slow,” you will have to wade into the innards (at least until AI can do that without mistakes).

If it’s software that will never be modified, sure it doesn’t matter.

And most users are not consuming your code. They’re consuming some compiled, transpiled, or minified version of it. But they do have expectations and it’s easier to amend the product if the source code is maintainable.